1. Field of the Invention
The present invention relates to a composite oxide thin film, and more particularly, to a composite oxide thin film formed through an electrochemical reaction and a water thermal reaction.
2. Description of Prior Art
Composite oxide thin films are attracting general attention as electronic materials for various applications and have already been industrialized or subjected to trial manufacture in different manners as materials for an inductor, a sensor, an optical component, a magnetic use and a superconducting application.
There have conventionally been known, as such composite oxide thin films, ones formed by physical evaporation as typically represented by sputtering and ones formed by chemical evaporation as typically represented by CVD and MOCVD. These conventional composite oxide thin films based on vapor synthesis involve some problems to be solved.
More specifically, these films based on vapor synthesis are defective in that they have a low rate of growth of the film and require consumption of much energy. In these methods, easy occurrence of non-uniform evaporation and the reaction under a low partial oxygen pressure tend to cause much oxygen demand, leading to the possibility of being converted into semiconductors, thus needing annealing after film formation. During annealing, however, the substrate and the composite oxide thin film may react, or peeloff may occur.
The low insulation fracture voltage relative to the film thickness is another problem.
In the case of the CVD method, a raw material of a high volatility must be used, but such a raw material is usually unstable and difficult to handle, with a very high cost.
In addition to these vapor phase methods, there are known several thin film forming methods based on the liquid phase process, including, for example, a method for forming a dielectric thin film by causing an electro-chemical reaction through immersion or titanium of zirconium in a molten salt of barium or strontium (Japanese Patent Publication No. 43-2,650), a method of immersing titanium in a molten salt (Japanese Patent Publication No. 44-13,455), and a method for forming a BaTiO.sub.3 film through a chemical treatment in a strongly alkaline aqueous solution of barium (Japanese Patent Provisional Publication No. 60-116,119).
In the methods using molten salt, however, it is necessary to employ a very high temperature and an expensive pressure vessel and contamination from the vessel is inevitable. It is furthermore difficult to precisely control the film thickness.
In the case of chemical treatments, the defects include the low growth rate and the difficult control of the film thickness, and in addition, there is a concern about contamination from such mineralizers as sodium and potassium. In addition to those mentioned above,the organic metal application method is known. This method is however defective in that the thermal decomposition through firing of an organic metal compound applied to the substrate at a prescribed temperature causes a considerable shrinkage during the firing step and produces cracks in the resultant composite oxide thin film, and furthermore, evaporation and combustion of the organic components make it difficult to achieve a dense sintar. The reaction with the substrate during firing is another problem.
The present invention was developed in view of the circumstances as described above and has an object to provide a new composite oxide thin film which solves the drawbacks of the conventional thin films, can be synthetically manufactured at a temperature lower than in the conventional manufacturing methods, is uniform and excellent in crystallinity, and easy to manufacture even in the case of a large-area film.